It is well known that droplets of water can condense from a humid atmosphere onto a relatively cool surface, e.g., as dew. See, for example, Woltman, H. L., "A Study of Dew and Frost Formation On Retro-Reflectors", Highway Research Record No. 70, National Academy of Sciences, 1965. Dew formation on signs typically occurs during periods of falling ambient temperature, e.g., during evening and nighttime, where objects such as signs tend to radiate heat and cool, causing the temperature of the objects to fall below the dew point of the surrounding air. Frost formation typically occurs under relatively cooler, but otherwise similar, conditions.
In the case of signs such as highway signs, particularly retroreflective signs, formation of dew or frost on the surface thereof may impair the visibility of the indicia thereon as the amount of light retroreflected by a retroreflective article is typically reduced by the presence of dew or frost thereon.
In FIGS. 1 and 2 of Huang U.S. Pat. No. 4,844,976, the loss of retroreflective brightness caused by formation of dew on the front surface of retroreflective signs is illustrated. That patent discloses application of a polymeric coating comprising silica and a transparent polymer to the front surface of retroreflective sheeting to increase soil and dew repellency. Huang U.S. Pat. No. 4,755,425 also discloses coatings which may be used on the front surfaces of retroreflective signs to impart greater dew repellency thereto. Funaki et al. U.S. Pat. No(s). 4,522,966, 4,594,379, and 4,642,266 disclose anti-fogging coating compositions that may be applied to the front surfaces of signs.
Generally, however, under conditions of very high humidity and/or rapidly falling ambient temperature, such coatings may not provide the desired degree of resistance to dew or frost formation.